The argument is: He cannot be tried for a different crime on the same facts. He could only be tried again for the same crime, but that couldn't happen in the scenario we are discussing because the ISC would have said that the crime in question doesn't exist. So there is an irreconcilable contradiction which can only be dealt with by the over turning of his conviction and letting him go.
As I understand it, such an application for review of trial goes first to the appellate court and not the ISC with the judgement being an administrative one. If he loses, he can appeal to the ISC. If he prevails in either arena, what is then supposed to happen in the process is that there is a retrial, heard at the appellate level (2nd instance), but the process does not appear to have taken account of the possibility of two ISC motivations being at odds with each other, thus preventing a retrial.
All this is moot if the ISC in the Knox/Sollecito motivation fudges the issue of multiple attackers or says there were multiple attackers but not Amanda and Raffaele.
But, it would also seem likely that Guede's application for review could be resisted by construing the motivation in his own case differently.
Nothing to do with this is particularly clear or pretty. As we all (or most of us) know, what should have happened is that Guede was charged and tried as the sole killer.
Rewind the clock to the middle of November 2007. Regardless of how the PLE/Mignini got to arresting AK, RS and PL with this murder and declaring, "case closed".......
...... imagine what they should have done when the "fourth suspect" was first identified. Lumumba was already on his way out of jail because it was simply ludicrous to arrest him in the first place
(and by that, I mean: ludicrous to arrest him on the info gleaned at an all night interrogation of a foreigner, when even the translator/mediator/diplomat Anna Donino couldn't even translate "See you later" properly).......
And none of the forensics was backing up the stellar investigative instincts of ANY of the cops.
Imagine taking to court in 2009 Rudy Guede as a sole killer of Meredith Kercher. Despite initial thoughts that the climb in Filomena's window was impossible, the cops had actually asked a kid to go up the wall to see if it was doable - just like Channel (5?) eventually did.....

Ok. That's how Rudy got in. Rudy was lying when he said Meredith let him in. Rudy was lying when he said he and the victim were having consensual sex, interrupted by his need to shed a kebob in the bathroom. Rudy was lying when he said he heard an argument between Meredith and one of her roommates, and Rudy was lying when he said he saw a man leaving, who'd said, "Black man found, black man guilty."
At the sole trial in 2009 of Rudy Guede they quickly identify that all the other DNA-stuff on the bra-clasp was contamination, probably Raffaele Sollecito, but also three other men, the whole thing suggesting contamination. The kicker though was that they DID test the semen stain, and found it to be Rudy's.
No one would know who some random Seattleite was, nor a kid from Bari, Italy. Winterbottom wouldn't have made a film, a film which essentially embarrasses an equally (now) unknown Barbie Nadeau.
Imagine how this should have proceeded when the "fourth suspect" was identified who actually had had a history of B&E's? There's be no Florence court appeal's judge (cf. Nencini) who'd say in an official court decision, that the reason Rudy had not staged the crime, was because it was exactly is own M.O. immediately recognizable by the PLE!!!!!!
Imagine if the lunacy of the last 7 1/2 years had been put to rest as soon as the fourth, and obvious suspect was identified and everyone else was sent home? The Kerchers would have had their answer in March 2013, when the ISC (as sane one) signed off on Rudy's obvious solo conviction.